Fluorides or hydrides? 41Ca performance at VERA’s 3-MV AMS facility

Recent improvements in isobaric suppression for medium-mass isotopes, e.g. 41Ca, offer new possibilities for tandem accelerators with terminal voltages of 3 MV or lower; i.e. when dealing with particle energies ⩽1 MeV/amu. In particular, detection of 41Ca requires sufficient discrimination of the stable isobar 41K. We explored the limits of 41Ca detection at our 3-MV AMS facility by means of different types of particle detectors: The ΔTOF method, which is based on the different flight-time of isobars after passing a thick absorber foil. The second method makes use of a new type of compact ionization chamber: 41K and 41Ca are separated in energy due to their different energy loss in the detector entrance foil and the detector gas, which is measured via a segmented anode.At VERA we measured 41Ca/Ca ratios below 10−13 for commercial CaF2 material serving as blank samples. CaH2 sputter targets, with the extraction of CaH3-, yielded background ratios as low as 41Ca/Ca = 1 × 10−15. The typical measurement precision at VERA for 41Ca measurements was between 2% and 5%. These results demonstrate that AMS facilities based on 3-MV tandems have reached the sensitivity level of larger AMS facilities for a wide range of applications, with the advantage of high overall efficiency and sample throughput.